This invention relates to a device for incrementally adjusting the position of the backrest member of a seat assembly, such as may be employed, for example, in automotive vehicle seat assemblies.
Known backrest adjustment devices typically comprise a first hinge bracket which is attached to the backrest member and a second hinge bracket which is attached to the seat member. The first hinge bracket is pivotably mounted with respect to the second bracket, and an adjustment means, actuated by a control handle, is typically provided for adjustment of the angle of the seatback member to the seat member.
One well known prior art adjustment device having this general arrangement utilizes harmonic gears running one within the other, the larger outer gear having one more tooth on its inner diameter than on the outer diameter of the smaller inner gear. The relative movement of the gears, which is controlled by a control handle operating through a drive shaft, results in a change in the inclination of the backrest member. The sizing and number of teeth of the harmonic gears is selected so as to generate sufficient gear friction to cause a self-locking action that is not overcome by the normal operating forces applied to the gears through the backrest member. Thus, it will be seen with this type of arrangement, that the adjustment means integrally incorporates a locking means to resist pivotal movement of the backrest member once the desired adjustment position is achieved. While this arrangement is simple and inexpensive to manufacture, it is unsatisfactory for a number of reasons. Such devices are inefficient or "sloppy" from a mechanical point of view, thus requiring considerable exertion by the user to overcome the inherent gear friction which holds the backrest member in the adjusted position. Further, because of this inefficiency, most of the occupant's weight must be removed from the backrest member before adjustment can be accomplished. By the nature of their design, these harmonic type of adjustment devices are subject to jamming and to premature wear-out. In addition, for satisfactory working specifications, the gear ratios must be such as to require a large number of turns of the control handle by the user to move the backrest member to a new desired position. This is both inconvenient and tiresome for the user.
Another known form of seatback adjustment device employs a ratchet and a pawl operationally connected one each to the seat member and the backrest member. A return spring biases the backrest member towards the upright "design" position and a handle is used to control the engagement of the pawl with the ratchet. When adjustment of the backrest member from the design position is desired, the handle is actuated to disengage the pawl. The operator, while holding the control handle in the disengaged position, uses his body weight and/or free hand to move the backrest member to the desired adjustment position, whereupon the control handle is released, causing the pawl to re-engage with the ratchet. With this arrangement, the backrest member is freely pivotable during the adjustment procedure and if the pawl fails to properly re-engage the ratchet after adjustment. This is undesireable from a safety standpoint as it may result in pivoting of the backrest member under rapid deceleration as encountered in crash situations.